Digital computer systems have a history of continually increasing the speed of the processors used in the system. As computer systems have migrated towards multiprocessor systems, sharing information between processors and memory systems has also generated a requirement for increased speed for the off-chip communication networks. Designers usually have more control over on-chip communication paths than for off-chip communication paths. Off-chip communication paths are longer, have higher noise, impedance mismatches, and have more discontinuities than on-chip communication paths. Since off-chip communication paths are of lower impedance, they require more current and thus more power to drive.
When using inter-chip high-speed signaling, noise and coupling between signal lines (cross talk) affects signal quality. One way to alleviate the detrimental effects of noise and coupling is through the use of differential signaling. Differential signaling comprises sending a signal and its compliment to a differential receiver. In this manner, noise and coupling affect both the signal and the compliment equally. The differential receiver only senses the difference between the signal and its compliment as the noise and coupling represent common mode signals. Therefore, differential signaling is resistant to the effects that noise and cross talk have on signal quality. On the negative side, differential signaling increases pin count by a factor of two for each data line. The next best thing to differential signaling is pseudo-differential signaling. Pseudo-differential signaling comprises comparing a data signal to a reference voltage using a differential receiver or comparator.
When high speed data is transmitted between chips, the signal lines are characterized by their transmission line parameters. High speed signals are subject to reflections if the transmission lines are not terminated in an impedance that matches the transmission line characteristic impedance. Reflections may propagate back and forth between driver and receiver and reduce the margins when detecting signals at the receiver. Some form of termination is therefore usually required for all high-speed signals to control overshoot, undershoot, and increase signal quality. Typically, a Thevenin's resistance (equivalent resistance of the Thevenin's network equals characteristic impedance of transmission line) is used to terminate data lines allowing the use of higher valued resistors. Additionally, the Thevenin's network is used to establish a bias voltage between the power supply rails. In this configuration, the data signals will then swing around this Thevenin's equivalent bias voltage. When this method is used to terminate data signal lines, a reference voltage is necessary to bias a differential receiver that operates as a pseudo-differential receiver to detect data signals in the presence of noise and cross talk.
The logic levels of driver side signals are determined by the positive and ground voltage potentials of the driver power supply. If the driver power supply has voltage variations that are unregulated, then the logic one and logic zero levels of the driver side signals will undergo similar variations. If the receiver is substantially remote from the driver such that its power supply voltage may undergo different variations from the driver side power supply, then additional variations will be added to any signal received in a receiver side terminator (e.g., Thevenin's network). These power supply variations will reduce noise margins if the reference has variations different from those on the received signals caused by the driver and receiver side power supply variations.
As the frequency of transmitted signals increases, the signal losses resulting from the signal propagating over a lossy transmission line also increase. These losses cause the high frequency content of the signal to attenuates and phase shifts relative to its lower frequency content. This results in receiver side signal distortion. This is especially true for signal transitions which may be slowed and distorted. Signal detection using pseudo differential signal generates an output by comparing a signal that arrives at a receiver to a reference voltage. As the high frequency content of a signal degrades, the signal transitions through the reference level more slowly and therefore the resulting detected signal has more uncertainty regarding timing of the signal transitions.
Signal quality may be increased by compensating for high frequency losses. It is well known that any repetitive signal or pattern may be decomposed in to its various Fourier frequency components. Such a decomposition of a generated signal and a signal received over a transmission line would show that high frequency content is attenuated more that lower frequency content. To compensate for these effects, one may decrease the low frequency content, boost the high frequency content or do both. Frequency compensation may be incorporated at the source or driver side, within the transmission network, or at the receiver side. Simply boosting the high frequency content may also increase the high frequency noise.
When using pseudo differential signaling to reduce pin count, it may be also desirable to generate a reference voltage for each data pin for improved receiver side power supply tracking. In this case, it would also be beneficial to apply controllable frequency compensation at the receiver side where each of the data nets may have different propagation losses. Data nets where the losses are not significant may suffer signal degradation if frequency compensation is added.
There is, therefore, a need for circuitry implementing receiver side high controllable frequency compensation for pseudo differential receivers.